1. Field of the Invention
The present invention relates to a speed change control method and a speed change controller. More specifically, the present invention relates to a speed change control method and a speed change controller for accurately controlling the output speed of an automotive or airborne traction-drive continuously variable transmission for an automobile, an aircraft, etc.
2. Description of the Related Art
A toroidal continuously variable transmission, which uses the inner circumference of a toroidal member as a friction surface, is one of traction-drive continuously variable transmissions (hereinafter referred to simply as “traction continuously variable transmissions” when necessary) for transmitting comparatively large power. In the toroidal continuously variable transmission, the tilt angle of a power roller rotating in contact with the respective friction surfaces of an input disk and an output disk is adjusted for the continuously variable control of speed change ratio.
Generally, a method of controlling the tilt angle of the power roller measures the tilt angle of the power roller or a parameter representing the tilt angle of the power roller and adjusts the tilt angle of the power roller by a local feedback control to improve control characteristic of speed change ratio control of such a toroidal continuously variable transmission, instead of measuring only output rotating speed and adjusting the tilt angle of the power roller.
FIG. 9 is a diagrammatic view showing the basic idea of a conventional speed change control.
Suppose that a power generating system 100 converts the rotative output 101 of an aircraft engine into a rotative driving force 104 of a fixed rotating speed by a continuously variable transmission mechanism 103 including a toroidal continuously variable transmission 102, and drives a generator 105 by the rotative driving force 104 to produce an alternate current of a predetermined frequency. The power generating system 100 employs a hydraulic cylinder actuator 111 as an actuator for adjusting the tilt angle of a power roller 110 included in the toroidal continuously variable transmission 102. The hydraulic cylinder actuator 111 shifts the position of the axis of rotation of the power roller 110 (hereinafter referred to as “power roller position”) to adjust the tilt angle of the power roller 110.
A control system controls the continuously-variable transmission mechanism 103. The control system includes a subtractor 109 which subtracts a measured rotating speed 107 of the output shaft that provides the rotative driving force 104 of the toroidal continuously variable transmission 102 from a desired rotating speed 106 of the output shaft of the toroidal continuously variable transmission 102 to obtain a rotating speed difference 108, a rotating speed controller 113 which generates a desired power roller position 112 specifying a power roller position for reducing the rotating speed difference 108 to zero, and a tilt angle control loop 114 which controls the tilt angle of the power roller 110 in a feedback control mode to make a measured power roller position 115 coincide with the desired power roller position 112.
The tilt angle control loop 114 includes a position subtractor 117 which subtracts the measured power roller position 115 from the desired power roller position 112 to provide a power roller position difference 116, and a position controller 119 which generates a desired valve opening signal 118 indicating a desired opening of a flow control valve, not shown, for controlling the flow of a working fluid into the hydraulic cylinder actuator 111.
Thus, the tilt angle control loop 114, i.e., a local feedback control loop, of the power generating system 100 controls the tilt angle of the power roller 110 on the basis of the measured power roller position 115 to control speed change ratio.
FIG. 10 shows a mechanism 200 proposed in Japanese Pat. No. 2568684 including a local feedback control system.
The control mechanism 200 measures a tilt angle change φg in the tilt angle of a power roller 201 mechanically by a precession cam, and changes power roller position to control the tilt angle of the power roller 201 in a feedback control mode. The control mechanism 200 includes a hydraulic cylinder actuator 202 for changing the power roller position, a flow control valve mechanism 203 for regulating the flow of a working fluid into the hydraulic cylinder actuator 202, and a tilt angle change measuring mechanism 204 for measuring the tilt angle change φg mechanically and adjusting the valve opening of the flow control valve mechanism 203.
The flow control valve mechanism 203 includes a sleeve 206 driven for axial displacement by a speed change motor 205, a spool 208 fitted in the bore of the sleeve 206 and urged by a spring 207 in a direction to increase the valve opening, and a port 209 opened and closed by the spool 208.
The control mechanism 200 provides a speed change signal indicating a desired speed change ratio for the toroidal continuously variable transmission 102 corresponding to an angular position for the output shaft of the speed change motor 205, and adjusts the valve opening of the flow control valve mechanism 203 according to the tilt angle change φg mechanically measured by the tilt angle change measuring mechanism 204. Thus, the flow of the working fluid supplied to the hydraulic cylinder actuator 202 is adjusted to provide a power roller position signal. The position 214 of a piston 213 included in the hydraulic cylinder actuator 202 is displaced according to the change of the flow of the working fluid to adjust the power roller position.
FIG. 11 shows another control mechanism 300 proposed in JP-A No. 257686/2000.
The control mechanism 300 employs a position sensor, such as a linear variable differential transformer (abbreviated to “LVDT”), for measuring a tilt angle change φg in the tilt angle of a power roller 301. The control mechanism 300 controls the tilt angle of the power roller 301 on the basis of a data provided by the position sensor 302 in a feedback control mode. The position sensor 302 measures the position of a piston 304 included in a hydraulic cylinder actuator 303 for adjusting the position of the power roller 301 to determine a power roller position. The position sensor 302 gives measured power roller position data 305 to a position controller 306. Then, the position controller 306 generates a valve-opening signal 309 indicating a valve opening for a flow control valve mechanism 308 on the basis of the power roller position data 305 and a speed change signal 307. The flow of the working fluid supplied to the hydraulic cylinder actuator 303 is adjusted according to the valve-opening signal 309 to adjust the power roller position.
Thus, there have been proposed various control systems that measures the power roller position mechanically or electrically and controls the tilt angle of the power roller dominating the speed change ratio on the basis of the measured power roller position to improve the control characteristic of the speed change ratio control of a traction continuously variable transmission.
A method of mechanically measuring the tilt angle change in the tilt angle of the power roller, as mentioned in the description of the control mechanism 200 proposed in Japanese Pat. No. 2568684, requires the effect of machining errors in the dimensions of the components of the measuring mechanism, assembly errors in the measuring mechanism, and errors attributable to play and backlash between the component parts on the accuracy of measurement to be limited below a level that will adversely affect the accuracy of measurement. When a precise speed change ratio control is necessary, the dimensional accuracies of the component parts of the measuring mechanism must be raised, and the component parts need very difficult machining and assembling work.
Since the mechanical measuring mechanism is an assembly of many accurate component parts, the measuring mechanism becomes unavoidably large and heavy. Such a measuring mechanism is unsuitable for use on an aircraft and is inevitably costly. The performance of the mechanical measuring mechanism is greatly affected by the deterioration of the accuracy of the component parts with time. Increase in play and backlash between the component parts with time and deformation of the component parts deteriorate the accuracy of control and entails the deterioration of the stability and performance of the power generating system.
The control mechanism 300 proposed in JP-A No. 257686/2000 employing the highly accurate position sensor, such as a LVDT, for measuring the power roller position is inevitably large and heavy. The control mechanism 300 is difficult to install when the same is applied to an aircraft power generating system, and such a highly accurate position sensor is expensive and increases the costs of the control mechanism 300 unavoidably.